Apparatus and method for use in non-invasively determining conditions in the circulatory system of a subject

ABSTRACT

Apparatus and method for providing breathing gases to a subject employs an exchanger taking up a quantity of a given component, such as CO 2 , from expiratory breathing gases passing through the exchanger and thereafter releasing the given component in inspiratory breathing gases subsequently passing through the exchanger. The exchanger may be selectively inserted in a flow path for the breathing gases for this purpose. Or, the breathing gases may be selectively passed through and bypassed around the exchanger. The apparatus and method may be used for non-invasive determination of the functional cardiac output of a patient using the differential form of the Fick equation.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to apparatus and method for use innon-invasively determining a condition of the circulatory system of asubject. More particularly, the present invention is directed to anapparatus and method for non-invasively determining the functionalcardiac output of the heart.

[0002] The physiological function of the heart is to circulate bloodthrough the circulatory system to the body and lungs. For this purpose,the heart receives blood in arterial chambers during its relaxed ordiastolic phase and discharges blood from its ventricle chambers duringthe contractile or systolic phase. The amount of blood discharged from aventricle chamber of the heart per unit time is the cardiac output (CO).A typical cardiac output for the heart of a normal adult (at rest) is5-6 liters per minute.

[0003] During circulation through the body, the blood is depleted ofoxygen (O₂) and is enriched with carbon dioxide (CO₂) as a result of themetabolic activity of the body. A major purpose for blood circulation isto take venous blood that has been depleted in O₂ and enriched in CO₂ asa result of its passage through the tissues of the body and supply it tothe lungs. In the alveoli of the lungs, O₂ is supplied to the blood fromthe breathing gases, typically air, and CO₂ is discharged into thebreathing gases. The oxygenated arterial blood is then supplied to thebody tissues. The gas exchange takes place in the capillaries of thelung because of the differences in concentration, or partial pressure,of O₂ and CO₂ in breathing gases, such as air, and in the venous blood.That is, the blood is low in O₂ and high in CO₂ whereas air is high inO₂ and low in CO₂.

[0004] A common condition reducing the gas exchange efficiency of thelungs is the presence of shunt perfusion or blood flow in the lungs. Ashunt comprises pulmonary blood flow that does not engage in gasexchange with breathing gases, due to blockage or constriction inalveolar gas passages, or for other reasons. This shunt blood flow thusbypasses normal alveoli in which gas exchange is carried out. Uponleaving the lungs, the shunt blood flow mixes with the non-shunt bloodflow. The former reduces the oxygen content and increases the CO₂content in the mixed arterial blood supplied to the body tissues.

[0005] It will be appreciated that only the non-shunt pulmonary bloodflow through the lungs participates in the gas exchange function of thelungs and in oxygenation and CO₂ removal in the blood of the subject.The quantity of blood that participates in such pulmonary gas exchangein the lungs is termed functional cardiac output (FCO). For diagnosticor other purposes, it is frequently desirable or essential to know thisquantity.

[0006] While shunt conditions can occur in the lungs due to blockagebrought about by disease, mechanical ventilation, particularly when therespiratory muscles of a subject are relaxed as during anesthesia, canresult in an increase in the pulmonary shunt. The breathing gasessupplied to the lungs can be enriched with oxygen under such conditionsto assist in oxygenation of the blood. However, a sufficient amount ofCO₂ may not be removed from the blood when the pulmonary shunt isincreased, giving rise to potentially adverse consequences to thesubject.

[0007] The classic technique for determining the functional cardiacoutput of the heart is through use of the Fick equation $\begin{matrix}{{FCO} = \frac{{VCO}_{2}}{{CvCO}_{2} - {CcCO}_{2}}} & (1)\end{matrix}$

[0008] where,

[0009] VCO₂ in ml/min. is the amount of CO2 released from the blood inthe circulatory system of the subject,

[0010] CvCO₂ is the mixed venous blood CO₂ content, for example in mlCO₂/ml of blood, and

[0011] CcCO₂ is the end capillary blood CO₂ content, i.e. the CO₂content in the blood leaving the ventilated lungs.

[0012] The Fick equation states that, knowing the amount of CO₂ gasreleased from the blood in a unit of time (e.g. the rate of gas transferas a volume/minute) and the concurrent gas transfer occurring per unitof blood (i.e. volume of gas/volume of blood), the blood flow throughthe lungs (i.e. FCO expressed in volume/minute) can be determined.

[0013] If a portion of the pulmonary blood flow of the subject is inshunt, this will decrease the amount of CO₂ released from the blood andthe computation of Equation (1) provides an indication of the resultingdecrease in functional cardiac output. In computing functional cardiacoutput using the Fick equation, the quantity VCO₂ can be determinednon-invasively by subtracting the amount of CO₂ of the inhaled breathinggases, for example air, from the amount of CO₂ of the exhaled breathinggases, taking into account changes in the amount of CO₂ stored in thelungs and the deadspace in the breathing organs of the subject, such asthe trachea and bronchi. The amount of CO₂ stored in the lungs can becomputed from the alveolar CO₂ gas concentration, as determined from anend tidal breathing gas measurement, and the end expiratory volumeV_(EE) of the lungs. The end capillary blood CO₂ content (CcCO₂) can bedetermined non-invasively, with a fair degree of accuracy, from ameasurement of the concentration of CO₂ in the breathing gases exhaledat the end of the expiration of a tidal breathing gas volume, i.e. theend tidal (ET) CO₂ level. See also Respiratory Physiology, by J. F.Nunn, published 1993 by Butterworths.

[0014] The venous blood CO₂ content (CvCO₂), is often determinedinvasively. An alternate non-invasive approach for the determination ofthe CvCO₂ can be seen in U.S. Pat. No. 6,042,550 and WO 01/62148. Inthese approaches, exhaled CO₂ enriched breathing gases are rebreathed bythe subject in subsequent inhalations. As rebreathing of the exhaledbreathings gases continues, breath-by-breath, the end tidal CO₂ partialpressure (P_(ET)CO₂) increases until the end capillary blood CO₂ partialpressure (P_(c)CO₂) is reached. At this point, it is postulated that theend tidal CO₂ partial pressure (P_(ET)CO₂), the alveolar CO₂ partialpressure (P_(A)CO₂), the end capillary blood CO₂ partial pressure(P_(c)CO₂), and the venous blood CO₂ partial pressure (P_(v)CO₂) are allequal and that this partial pressure can be converted to the venous CO₂content (C_(v)CO₂) for use in the Fick equation.

[0015] The need for the determination of the venous blood CO₂ content(C_(v)CO₂) is eliminated by the use of a differential form of the Fickequation which arises from the following circumstances. As a subjectrebreathes exhaled breathing gases, the end tidal CO₂ partial pressure(P_(ET)CO₂) and thus the alveolar CO₂ partial pressure (P_(A)CO₂) andend capillary CO₂ content increases. This reduces the venousblood-alveolar CO₂ partial pressure differences and because this is thedriving force for CO₂ elimination in the lungs, CO₂ elimination is alsoreduced. It has been shown that the ratio of the change in CO₂elimination to the change in the end capillary blood CO₂ content isequal to the functional cardiac output. See Gedeon A., et al. Med. Biol.Eng. Comp. 18:411-418 (1980). It is set forth in equation form, asfollows: $\begin{matrix}{{FCO} = {\frac{{VCO}_{2}^{N} - {VCO}_{2}^{R}}{{CcCO}_{2}^{R} - {CcCO}_{2}^{N}} = \frac{\Delta \quad {VCO}_{2}}{\Delta \quad {CcCO}_{2}}}} & (2)\end{matrix}$

[0016] In the differential form of the Fick equation, the superscript Nindicates values obtained in “normal” breathing conditions. Thesuperscript R indicates values obtained during a short term “reduction”in the CO₂ partial pressure difference between that in the alveoli andthat in the blood. This results in reduced CO₂ transfer in the lungs.

[0017] In using the differential form of the Fick equation, a first setof values for VCO₂ and CcCO₂ are obtained, as in the manner describedabove, under normal breathing conditions. These are identified by thesuperscript N. Thereafter, the amount of CO₂ in the breathing gases forthe subject is increased. This maybe accomplished by a partialre-breathing of exhaled breathing gases. See U.S. Pat. Nos. 5,836,300 or6,106,480 and published International Patent Appln. WO 98/26710 thatemploy valve mechanisms, to vary the re-breathed gas volume, for thispurpose. Or, this may be accomplished by injecting CO₂ into the inhaledbreathing gases as described in U.S. Pat. No. 4,608,995. Furtherpossibilities for altering the alveolar CO₂ content include varying lungventilation. This may be accomplished by altering the tidal volume orthe respiration rate. Single breath maneuvers such as a deep breath aspresented by Mitchell R R in Int J Clin Mon Comp 5:53-64 (1988),inspiratory hold as presented in WO 99/25244, or expiratory hold, mayalso be used for the purpose.

[0018] The CO₂ enrichment increases the concentration of CO₂ in thealveoli in the lungs and reduces the CO₂ partial pressure differencebetween that of the breathing gases in the lungs and that in the venousblood. As noted above, it is that CO₂ partial pressure difference thatdrives the CO₂ gas transfer from venous blood to the breathing gases inthe alveoli of the lungs. The reduced CO₂ partial pressure differencereduces CO₂ gas transfer in the lung and causes an elevation of the CO₂content in the blood downstream of the lung, i.e. in the arterial bloodof the subject. In the time interval before the blood with elevated CO₂content circulates through the body and returns to the lungs, the CO₂content of venous blood (CvCO₂) entering the lungs can be taken to bethe same for both the initial, normal breathing conditions (N) and thesubsequent, reduced CO₂ partial pressure difference conditions labeledby the superscript R. This similitude permits the factor CvCO2 to bedropped out of the Fick equation when expressed in the differential formas Equation 2 so that the cardiac output is determined by the ratio ofthe change in released CO₂ amounts (VCO₂) between the normal (N) andreduced (R) gas exchange conditions to the corresponding change in theend capillary blood CO₂ content (CcCO₂) in the normal and reduced (R)gas exchange conditions. The need to determine the venous blood CO₂content (CvCO2) from the subject is thus eliminated.

[0019] The foregoing approach is also advantageous with ventilated oranesthetized subjects since the alteration of the CO₂ content of thebreathing gases can be effected by altering the ventilation provided tothe subject. In the case of a subject anesthetized with a breathingcircuit of the recirculating type, the alteration in CO₂ content may becarried out by bypassing the CO₂ absorber in the breathing circuit. TheCO₂ absorber removes CO₂ from exhaled breathing gases of the subjectthereby allowing the breathing gases to be recirculated to forminspiratory breathing gases for the subject. Bypassing the absorberincreases the amount of CO₂ in the breathing gases that are recirculatedto the subject for inspiration.

[0020] While the above described techniques avoid the need to invasivelydetermine venous blood CO₂ content, other problems are created. In casesin which a subject is being provided with a fixed volume of breathinggases, an increased re-breathing volume is accompanied by a decreasedvolume of inspired oxygen. This may produce an undesired reduction inthe oxygen content in the blood or require increased oxygenconcentrations in the inspired breathing gases, following a cardiacoutput measurement, to restore oxygen levels in the blood to desiredvalues. Also the tubing required for the large re-breathing volume addsto the size of associated valve systems making them big and bulky whenassembled at the very crowded area near the mouth and nose of thesubject. Such apparatus also adds to the overall ventilation dead-spacevolume between the breathing circuit for the subject and the subjectslungs. This increases the amount of ventilation required, adding to therisk of lung distension.

[0021] The injection of carbon dioxide into inspired breathing gasovercomes the problems of reduced oxygenation and bulky valve systems,but raises analogous problems. The CO₂ is obtained from a gas source andis typically injected using a gas tube. Such a tube is not normallypresent at the point of care for the subject and adding such a tube,with the accompanying high-pressure regulators and supply conduits, intothe already crowded care environment is also undesirable.

BRIEF SUMMARY OF THE INVENTION

[0022] An object of the present invention is to provide an improvedapparatus and method for carrying out an alteration in the CO₂ contentof breathing gases inspired by a subject for purposes of non-invasivelydetermining a circulatory system condition, e.g. the functional cardiacoutput, of a subject.

[0023] Another object of the present invention is to provide anapparatus and method that can carry out such alteration withoutaffecting the exchange of other respiratory gases, such as oxygen, inthe lung.

[0024] Yet another object of the present invention is to provide suchapparatus that minimizes disturbance to a patient care environment andminimizes the overall increase in the breathing circuit-lung dead-spacevolume.

[0025] Briefly, in accordance with the improved apparatus and method ofthe present invention for altering the CO₂ content of the breathinggases, and the lung CO₂ partial pressure, the breathing gas flow isselectively guided through a CO₂ exchanger in a flow path for thebreathing gases. The CO₂ exchanger selectively takes up CO₂ from theexpired breathing gases of the subject and releases it to the breathinggases inhaled in a subsequent inspiration. Such an exchanger can be madeof a gas porous element, for example, activated charcoal or zeolite,with pore sizes suitable for the adsorption CO₂.

[0026] The CO₂ exchanger can be in a form of a moveable element, thatcan, with the aid of a transfer mechanism, be moved into and out a flowpath of the breathing gases. Alternatively, especially during prolongedartificial ventilation of a subject in intensive care, when the dryinspiration breathing gas is often humidified and warmed with a heat andmoisture exchanger (HME), the CO₂ exchanger can be connected in parallelwith such an HME. Using a control valve, the breathing gas flow can bedirected either through the HME, thereby forming a CO₂ exchanger bypasschannel, or through the CO₂ exchanger. With such an arrangement, anincrease of the dead space in the breathing gases pathway is avoided.The temporary interruption of the humidification when the breathing gasis directed through the CO₂ exchanger is easily tolerated by thesubject. To keep the gas exchange conditions unchanged gases other thanCO₂, the volume of the CO₂ exchanger and associated components isadvantageously equal to the volume of the by-pass channel containing theHME.

[0027] Breathing gas measurements obtained when the breathing gases arenot passing through the exchanger and when they are passing through theexchanger may be used to determine the functional cardiac output of thesubject using the differential Fick equation, in the manner describedabove.

[0028] Various other features, objects, and advantages of the inventionwill be made apparent from the following detailed description and thedrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0029] In the drawing:

[0030]FIG. 1 is a graph showing data obtained from the breathing gasesof a subject under normal breathing conditions and under conditions ofreduced gas exchange in the lungs of the subject;

[0031]FIG. 2 shows a breathing device using the apparatus of the presentinvention in order to determine functional cardiac output;

[0032]FIG. 3a is a detailed cross sectional view of the apparatusaccording to the present invention showing a moveable CO₂ exchangerelement in a position in which the breathing gases of the subject bypassthe CO₂ exchanger element;

[0033]FIG. 3b is a similar view showing the CO₂ exchanger elementtransferred to a position in which it is in the breathing gas flow path;

[0034]FIG. 4 is a graph of the breathing gas CO₂ concentration when thebreathing gas is passed through the CO₂ exchanger element and when itby-passes the exchanger element; and

[0035]FIG. 5 is an alternative embodiment of the CO₂ exchanger apparatusof the present invention connected in parallel to a heat and moistureexchanger.

DETAILED DESCRIPTION OF THE INVENTION

[0036] The basic principles of the analytical technique in which theapparatus and method of the present invention find use are as follows.For one or more normal (N) breaths of the subject, values are obtainedfor the amount of CO₂ released from the blood (VCO₂ ^(N)) and for aquantity indicative of the end capillary blood CO₂ content, for exampleCcCO₂ ^(N). One or more values for the same quantities are obtainedunder conditions of reduced (R) gas exchange in the lungs of thesubject, to comprise VCO₂ ^(R) and CcCO₂ ^(R) values. This isaccomplished by enriching the inspired breathing gases with CO₂. Thebreathing gases are then, again, returned to the normal condition.

[0037] The normal (N) breathing values (N) and reduced (R) gas transfervalues (R) are used as data points for a regression analysis, such as alinear regression analysis. Graphically, the data points may be plottedon a graph in which the end capillary CO₂ blood quantity values, such asCcCO₂, are scaled along the abscissa and values for the released amountof CO₂ (VCO₂) are scaled along the ordinate. Such a graph is shown inFIG. 1. For simplicity only, a single set of N and R data points areshown in FIG. 1 as points 10 and 12, respectively. The regressionanalysis produces a straight line 14 providing the best fit for the datapoints. In the simplified example shown in FIG. 1, this is a straightline intersecting the two data points. The downward slope of line 14makes it clear that the greater the amount of CO₂ that is released inthe exhalations of the subject, the less will be the end capillary bloodCO₂ content of the subject.

[0038] It will also be appreciated that the slope of line 14 representsthe functional cardiac output of the subject as expressed in thedifferential form of the Fick equation, Equation 2. That is, thedifference between the amount of CO₂ (VCO₂) released under normal (N)conditions and that released under reduced (R) gas transfer conditionsshown along the ordinate of FIG. 1 represents the numerator of Equation2. The corresponding situation exists with respect to the difference inend capillary blood CO₂ content (CcCO2) shown on the abscissa of FIG. 2and forming the denominator of Equation 2. When Equation 2 is presentedgraphically in the manner shown in FIG. 1, the functional cardiac outputthus determined will have a negative sign due to the transposition ofthe quantities forming the denominator of the equation.

[0039]FIG. 2 shows a device suitable for incorporating the apparatus ofthe present invention and carrying out the method of the presentinvention. The breathing organs of the subject, including lungs 20 aresupplied with breathing gases through breathing circuit 22 ofconventional construction. Breathing circuit 22 includes inspirationlimb 24 that supplies breathing gases to the subject and expiration limb26 that receives exhaled gases from the subject. Inspiration limb 24 andexpiration limb 26 are connected to two arms of Y-connector 28. A thirdarm of Y-connector 28 is connected to patient limb 30. Patient limb 30supplies and receives breathing gases to/from the subject through anendotracheal tube, face mask, or other appliance (not shown).

[0040] The other ends of inspiration limb 24 and expiration limb 26 areconnected to ventilator 32. Ventilator 32 provides breathing gases ininspiration limb 24 and receives breathing gases from expiration limb26.

[0041] The patient limb accommodates also a flow sensor 34 connectedthrough a signal line 36 to the monitor 38. A flow measuring apparatussuitable for use in breathing circuit 22 is shown in U.S. Pat. No.5,088,332 to Instrumentarium Corp. of Helsinki, Finland. A hot wireanemometer may also be used for this purpose. The flow sensor may alsobe placed elsewhere in the circuit than at the location shown in FIG. 2.A CO₂ sensor 40 is also located at the patient limb. This sensor can beof mainstream type when the signal line 42 is an electrical one and theactive sensor element, typically based on infrared light absorption, ismeasuring the gas flow in the patient limb. Alternatively, the CO₂sensor 40 may be of sidestream type, when the element in the patientlimb is a sampling port and the line 42 is a sampling line conveying asample gas flow to the infrared analysis within the monitor 38. The CO₂sensor is used to determine the end-tidal CO₂ concentration and,together with the flow signal from flow sensor 34, is used to determinethe CO₂ elimination from the lungs by integrating the product ofinstantaneous flow and the corresponding CO₂ concentration.

[0042] The output of sensors 34 and 40 are provided in signal lines 36and 42 to monitor 38 in which the integration of flow rates to obtainvolumes, filtering, or other signal processing is carried out to producevalues for the sensed quantities.

[0043] Sensors 34 and 40 and monitor 38 measure gas flows, expired CO₂concentrations, and end tidal CO₂ gas concentrations. Measured expiredCO₂ concentrations and gas flows can be used to determine the amount ofCO₂ (VCO₂) released from the blood. The end tidal CO₂ concentration isused to determine quantities indicative of the CO₂ content of the blood,such as CcCO₂, as described above.

[0044] As shown in FIG. 2, the CO₂ exchanger apparatus 50 of the presentinvention is located in the patient limb 30. One embodiment of theexchanger apparatus is shown in FIGS. 3a and 3 b. CO₂ exchangerapparatus 50 has housing 52 with ports 54 and 56 for connecting the CO₂exchanger apparatus in patient limb 30, as shown in FIG. 2. As shown inFIG. 2, CO₂ exchanger apparatus 50 is connected in patient limb 30upstream of CO₂ sensor 40. That is, CO₂ sensor 40 is positioned betweenCO₂ exchanger apparatus 50 and the subject, i.e. subject's lungs 20.Housing 52 of CO₂ exchanger apparatus 50 includes a moveable element 58containing a substance capable of taking up a quantity of CO₂ fromexpiration breathing gases passing through the element and thereafterreleasing the taken up quantity of CO₂ to inspired breathing gasessubsequently passing through the element. For this purpose and by way ofexample, element 58 may comprise a porous housing 60 containingactivated charcoal rods. Such a material adsorbs the CO₂ from the highCO₂ partial pressure expiration breathing gases, and due to the weaknessof the bonding of the CO₂ to the absorption material, thereafterreleases or relinquishes the CO₂ to the low CO₂ partial pressureinspiration breathing gases. The two-way taking up and releasing actionof the CO₂ exchanger of the present invention distinguishes it from aCO₂ absorber conventionally found in recirculating breathing circuits.The function of a CO₂ absorber is to permanently remove CO₂ from thebreathing gases of a patient. The activated charcoal rods may, forexample, be 1 mm in diameter and 1-5 mm in length. A typical volume ofmaterial for taking up CO₂ and releasing a sufficient quantity toadequately increase the alveolar CO₂ partial pressure is 10-30 ml,depending the exact geometry of apparatus 50 and element 58. For anapparatus suitable for pediatric patients the volume of CO₂absorption/release material may be smaller. Other materials, such aszeolite with pore sizes suitable for the adsorption of CO₂ may also beused.

[0045] Element 58 may be moved from a position which is shown as anupper position in FIG. 3a, to a lower position shown in FIG. 3b. In thesimplest embodiment of the invention, a manual actuator 64 may beemployed as a transfer mechanism for this purpose. In a typical,practical embodiment of the present invention shown in FIG. 2, manualactuator 64 is replaced with an electrical solenoid or linear motor 66operable by a signal in line 68 from monitor 38. It would also bepossible to provide a pneumatic actuator in apparatus 50.

[0046] With element 58 in the raised, upper position shown in FIG. 3a,breathing gases to/from the patient proceed directly between ports 54and 56 of housing 52 of apparatus 50. With element 58 in the loweredposition, shown in FIG. 3b, breathing gases passing between ports 54 and56 pass through element 58 and the gas take up/release substance 62. Aseal 67 may be provided in the lower portions of housing 52 toaccommodate element 58 when it is in the lowered position.

[0047] The method for carrying out the method of the present inventionis as follows. The method is described as in an instance using air forthe breathing gases. Respiration may be either spontaneous on the partof the subject or assisted by the ventilation apparatus shown in FIG. 2.

[0048] Element 58 of apparatus 50 is placed in the upper position shownin FIG. 3a. The subject breathes, or is ventilated, with breathing gasessuch as air. The normal (N) breathing action of the subject is allowedto stabilize. This may, for example, require a minimum of five breathsor a half a minute to a minute of time. The amount of CO₂ released fromthe blood in the lungs of the subject and the CO₂ concentration in thebreathing gases are then measured, for at least one breath, orpreferably for each of a plurality of breaths, of the subject usingsensors 34 and 40. Typically, the CO₂ concentration is measured as theend tidal CO₂ concentration (P_(ET)CO₂ ^(N)). One or more values of VCO₂(N) are determined. In this exemplary description, the quantity used todescribe the end capillary blood CO₂ condition is the CO₂ content(CcCO₂). The measured end tidal CO₂ concentrations are thus used todetermine CcCO₂ and one or more CcCO₂ N values are obtained from the endtidal CO₂ levels for the breaths.

[0049] Thereafter, the CO₂ content of the breathing gases inhaled by thesubject is increased to increase the CO₂ concentration in the lungs ofthe subject and to reduce CO₂ gas transfer, i.e. (R) breathingconditions. Using the apparatus shown in FIG. 3a, this may beaccomplished by lowering element 58 to place the element in thebreathing gas flow path between ports 54 and 56, as shown in FIG. 3b.

[0050] The end tidal CO₂ levels are examined as the subject breatheunder these conditions. FIG. 4 shows a read out of the CO₂ levels of thebreathing gas passing CO₂ sensor 40 downstream of apparatus 50. Prior totime 70, element 58 in apparatus 50 is in the raised position so thatthe breathing action of the subject is in the normal (N) one describedabove. For each breath, the CO₂ level starts at essentially zero duringinhalation and rises to about 5% in the exhaled breathing gases.

[0051] At time 70, element 58 is lowered into the breathing gas passagebetween parts 54 and 56. Element 58 commences its CO₂ taking up andreleasing action. This causes the CO₂ content of the inhaled breathinggases to rise to over 1% and the CO₂ content of the exhaled breathinggases to increase to about, or over, 6%, as shown in FIG. 4. The resultis an increase in the inspired CO₂ content of about 1.0% which isconsidered optimal in carrying out the determination of functionalcardiac output.

[0052] When the end tidal CO₂ levels no longer change, this indicatesthat the alveolar CO₂ concentration in the lungs is constant, whichmeans that CO₂ storage in the lungs has been accommodated. Themeasurement of the amount of gas released from the lungs of the subjectand CO₂ concentrations of the breathing gases, i.e. end tidal CO₂concentration, is then commenced. After measurements are taken, theenrichment of CO₂ in the inhaled breathing gases may thereafter beterminated by raising the CO₂ take up/release element 58 to the upperposition shown in FIG. 3a at time 72.

[0053] The exact amount and duration of the CO₂ enrichment will dependon numerous physical and physiological factors of the patient and on thedata needed to accurately determine functional cardiac output. For atypical adult, CO₂ enrichment would last about 6 or 10 breaths.

[0054] The amount of end-tidal CO₂ increase is governed by somewhatconflicting considerations. The larger the increment, the larger will bethe alveolar CO₂ concentration in the lungs and the end capillary bloodCO₂ content (CcCO₂). This will place the R data point 12 farther fromthe abscissa of FIG. 1 and improve the accuracy of the FCOdetermination. On the other hand, the larger the CO₂ increase is, theless CO₂ gas exchange occurs in the lungs of the subject resulting inhigher CO₂ blood levels that require a longer time to return to normallevels. The optimum of CO₂ increase a combination of these factors andneed be no greater than that required to achieve the desired results.

[0055] The amount of CO₂ released from the blood of the subject (VCO₂R)is determined by subtracting the amount of CO₂ in the enriched, inhaledbreathing gases from the CO₂ amount measured in the exhaled breathinggases. The measured end tidal CO₂ levels are used to determine the endcapillary blood CO₂ content CcCO₂ ^(R). These determinations are carriedout from measurements obtained within the circulation period of theblood in the body of the subject following the switching of actuator 64,66 to transfer the CO₂ take up/release element 58 into the breathing gasflow path. This is a period of approximately 20 seconds to one minute.In this period, the venous blood CO₂ content (CvCO₂) remains constantsince it has not yet returned to the lungs to undergo gas exchange.

[0056] If desired, an administration of increased CO₂ in the inhaledbreathing gases to the subject can be repeated after an appropriateinterval during which CO₂ levels in the blood return to normal.

[0057] A regression analysis, such as a linear regression analysis, isthen performed using the normal (N) values obtained from the initialbreaths of the patient prior to time 70 in FIG. 40 and the reduced (R)gas transfer values obtained following the increase in the CO₂ contentof the inhaled breathing gases, i.e. after time 70. It will beappreciated that the data used to perform the regression analysis caninclude many normal (N) values obtained from the plurality of normalbreaths taken by the patient. There will be a smaller number of R valuesdue to the time limitation set by the blood recirculation.

[0058] As noted above, the slope of line 14 produced by the regressionanalysis is the negate of the functional cardiac output (FCO) of thepatient.

[0059]FIG. 5 presents an alternate embodiment in which the CO₂ take theCO₂ up/release element is positioned in parallel with a heat andmoisture exchanger (HME). Specifically, apparatus 501 contains CO₂ takeup/release element 581. Element 581 may be similar in construction toelement 58 except that it is not moveable in the housing 502 ofapparatus 501. Housing 502 contains ports 504 and 506. Part 504 may beconnected in patient limb 30. Part 506 is connected to valve 80.

[0060] Heat and moisture exchanger 82 is connected in parallel withapparatus 501 between patient limb 30 and valve 80. Valve 80 is alsoconnected to patient limb 30. Heat and moisture exchanger 82 may be ofconventional construction and includes a component 84, schematicallyshown in FIG. 5, for carrying out its intended purpose.

[0061] By the appropriate operation of valve 80, the breathing gases ofthe subject can bypass apparatus 501 and pass through heat and moistureexchanger 82, as prior to time 70 and subsequent to time 72, or passthrough apparatus 501, as between timer 70 and 72.

[0062] It is preferable that the volumes of the apparatus 501 and itsassociated flow paths and the volume of heat and moisture exchanger 82and its associated flow paths be made essentially equal to avoid changesin the gas exchange of gases other than CO₂. An adult heat and moistureexchanger is typically 40 ml by volume, and for pediatric patients thevolume may be 15 ml.

[0063] It is recognized that other equivalents, alternatives, andmodifications aside from those expressly stated, are possible and withinthe scope of the appended claims.

1. Apparatus for altering the amount of a given component in breathinggases provided to a subject, the subject breathing in respiratory cycleseach having an inspiration phase in which inspiratory breathing gasesare provided to the subject and an expiration phase in which the subjectexhales expiratory breathing gases, the given component being present inthe expiratory breathing gases of the subject, said apparatuscomprising: a conduit means having a flow path for providing inspiratorybreathing gases to the subject during the inspiratory phases of therespiratory cycles and receiving expiratory breathing gases from thesubject during the expiratory phases of the respiratory cycles; a gascomponent exchanger for taking up a quantity of the given component fromgas passing through said exchanger and releasing the given component ingas passing through said exchanger; and means for selectively passinginspiration and expiration breathing gases in said conduit through saidexchanger; the exchanger taking up the given component from theexpiratory breathing gases in an expiratory phase and thereafterreleasing the component into the inspiratory breathing gases in aninspiratory phase to raise the concentration of the component in theinspiration breathing gases provided to the subject.
 2. The apparatus ofclaim 1 wherein said means for selectively passing inspiratory andexpiratory breathing gases through said exchanger comprises means forselectively inserting said exchanger in said flow path of said conduitmeans.
 3. The apparatus of claim 1 wherein said means for selectivelypassing inspiratory and expiratory breathing gases through saidexchanger comprises means for selectively passing the breathing gasesthrough said exchanger or diverting the breathing gases from saidexchanger.
 4. The apparatus of claim 3 wherein said means for passing ordiverting the breathing gases includes an alternative flow path for thebreathing gases containing a gas treatment device.
 5. The apparatus ofclaim 4 wherein said gas treatment device comprises a heat and moistureexchanger.
 6. The apparatus of claim 4 wherein the volume of said flowpath and exchanger and the volume of said alternative flow path and gastreatment device are substantially the same.
 7. The apparatus of claim 3further including valve means for selectively passing or diverting thebreathing gases.
 8. The apparatus of claim 1 wherein said givenbreathing gas component is CO₂.
 9. The apparatus of claim 1 wherein saidexchanger includes activated charcoal for taking up and releasing thegiven component in breathing gases passing through the exchanger. 10.The apparatus of claim 1 wherein said exchanger includes zeolite fortaking up and releasing the given component in breathing gases passingthrough the exchanger.
 11. The apparatus of claim 1 further including aventilator coupled to said conduit means for supplying inspiratory gasesto the subject and receiving expiratory gases from the subject.
 12. Theapparatus of claim 11 further including a flow meter for measuring theflow of breathing gases.
 13. The apparatus of claim 11 further includinga breathing gas component measuring means between said exchanger and thesubject.
 14. A method for altering the amount of a given component inbreathing gases provided to a subject, the subject breathing inrespiratory cycles each having an inspiration phase in which inspiratorybreathing gases are provided to a subject and an expiration phase inwhich the subject exhales expiratory breathing gases, the givencomponent being present in the expiratory breathing gases of thesubject, said method comprising the steps of: passing expiratorybreathing gases of the subject along a flow path; taking up a quantityof said given component from expiratory breathing gases passing in theflow path; and thereafter releasing the component taken up into theinspiratory breathing gases to raise the concentration of the componentin the inspiratory breathing gases for the subject.
 15. The method ofclaim 14 further including the step of selectively inserting anexchanger into the flow path for the breathing gases, the exchangertaking up a quantity of the given component from expiratory breathinggases in the flow path and releasing the given component in inspiratorybreathing gases in the flow path.
 16. The method of claim 14 furtherdefined as including the step of selectively passing breathing gases orbypassing the breathing gases around the exchanger, the exchanger takingup a quantity of said given component from expiratory breathing gasespassing through the exchanger and releasing the given component ininspiratory breathing gases passing through the exchanger.